The present invention relates to a process for producing an isotope of an element and more particularly to isotopes of hydrogen and oxygen, specifically deuterium (D2) and 18O.
Deuterium is a naturally occurring, stable isotope of hydrogen. Deuterium is found in very low concentrations in the environment; its concentration is only 150×10−6 atomic fraction (natural abundance). The nucleus of deuterium contains one additional neutron, which results in differences in the physical and atomic properties when compared to hydrogen.
Deuterium can be made by the following exchange reaction: 2(HD)=H2+D2.
Deuterium is useful in the following applications:
1. Replacement of hydrogen in any molecule due to the same electronic structure as hydrogen; deuterium is often used as a tracer atom to discern the reaction mechanism for chemical reactions. See Melander et al., Reaction Rates of Isotopic Molecules, Wiley, New York (1980).
2. Nuclear fuel in fusion nuclear reactors.
3. Preparation of heavy water; deuterated water (D2O) is used in heavy water nuclear reactors. Deuterated water has less of a tendency to slow down neutron emitted during the fission reaction, thereby allowing the use of lower purity uranium fuel.
4. Replacement of hydrogen in annealing the silicon/silicon oxide gate interface in integrated circuits as described in WO 94/19829 to Lisenker et al. The bond dissociation energy of the Si-D bond is 72.3 Kcal/mol as compared to 71.5 Kcal/mol for the Si—H bond. This slight difference in bond energy is enough to prevent bond dissociation in the presence of high-energy electrons at the silicon/silicon oxide interface. This leads to substantially longer device lifetime for circuits annealed in deuterium versus those annealed in hydrogen.
5. Annealing optical fibers. The presence of O—H in glass fibers leads to substantial loss of signal in the near IR (1.3 to 1.55 μm). Fibers annealed in deuterium have O-D bonds, which are transparent in this region of the electromagnetic spectrum.
Isotopes can be produced by a variety of methods. One such method is a reactive separation process, which utilizes isotope-exchange-reaction equilibrium between gas and liquid components to affect isotope enrichment. The Girdler-Sulfide (GS) process is the most common method of deuterium enrichment to produce heavy water. The process utilizes the reaction between water and H2S as described by Rae, H. K. (1978), “Selecting Heavy Water Processes” in Separation of Hydrogen Isotopes, ACS Symposium Series 68, ACS, Washington, pp. 1–26. This process requires high liquid and gas flowrates (i.e. large vessels). In addition, the process requires handling corrosive streams.
Other reactive separation processes utilize H2/D2 gas mixtures reacting with ammonia, methylamine, or water in the presence of a catalyst to facilitate the exchange reaction. The exchange rate constant, even in the presence of the catalyst, is a factor of 10 to 100 times slower than the GS process. Thus, these reactive separation processes are less useful.
Another method for producing isotopes uses inorganic gas separation membranes comprising Pd at elevated temperatures. See, Sanchez et al., “Current Developments and Future Research in Catalytic Membrane Reactors,” in Fundamentals of Inorganic Membrane Science and Technology Ed. by A. J. Burggraaf and L. Cot, Elsevier, Amsterdam, pp. 529–568 (1996); Suzuki et al., J. At Energy Soc. Jpn., 26 (1984), pps. 802 and 999; Suzuki et al., Nuclear Technology, 103 (1993), pp. 93–100.
Isotopes can also be produced by cryogenic processes.
U.S. Pat. No. 4,353,871 to Bartlit et al. describes a cryogenic distillation process for the separation of a mixture of hydrogen, D2 and T2 in which the concentration is approximately 50% D2, 50% T2 and 1% H2. The system consists of four cryogenic distillation columns and two catalytic reactors for isotope exchange at room temperature. In the process, the overhead product containing HD is removed as waste, while the bottom output, HT and D2, are removed and sent to a second reactor.
Embury et al., AlChE Symposium Series (251), vol. 82 (1986), pp. 13–18 described a cryogenic distillation process for recovering three isotopes of hydrogen from a mixed feed. The process is used to recover tritium from nuclear reactor waste streams. The process consists of three interconnected distillation columns and two catalytic isotope exchange reactors. The distillation columns are operated at about 24° K., and the exchange reactors filled with a platinum catalyst are operated at 300° K. Embury et al. does not disclose using a reactive distillation or a cryogenic exchange reaction.
Clusius et al., Z. Naturforsch, 4 A: 549 in Nuclear Chemical Engineering (1949) describe feeding cold hydrogen to a primary column and concentrate HD to 5%–10%. The HD-free hydrogen distillate is compressed and returned to the first column as reflux after first being used as a heat source for the reboilers. A smaller double column downstream purifies the HD in the bottom of the upper column in preparation for the exchange reaction. The HD exchange reaction was done in a separate fixed bed at room temperature. The reactor effluent is fed to the lower column where D2 is recovered as a bottom output and the H2—HD distillate is recycled back to the upper column. Several shortcomings of this process are inability to produce liquid H2 and inability to use structured packing instead of trays in the distillation column.
Similar pilot plant experiments concentrating HD from natural hydrogen feed are further described. See, Timmerhaus et al., “Low Temperature Distillation of Hydrogen Isotopes”, Chem. Engr. Prog., 54(6) (1958), pp. 35–46; Timmerhaus et al., Cryogenic Process Engineering, Plenum Press, (1989), pp. 358–359. However, none of the above references disclosed a process wherein both the distillation and the exchange reaction are conducted at cryogenic temperatures.
The process disclosed in Timmerhaus et al. (1989) does not use a deuterium-depleted reflux in the distillation column. This results in deuterium losses due to immediate flash of the feed on the top tray. In addition, the overhead of the first column is used to condense the feed to the column, so that only gaseous hydrogen is produced. The feed is partly expanded to provide the reboiler duty to the first column and partly used to provide the reboiler duty in the D2-sump of the second column.
Kanda et al., “Experimental Research on the Rectification of Liquid Hydrogen to Obtain Deuterium” in Proceedings of the Second United Nations International Conference on the Peaceful Uses of Atomic Energy 4:550 (1958) uses hydrogen as the cooling fluid and the primary column only achieves 90%–95% yield of HD from H2.
U.S. Pat. No. 3,216,800 to Stouls describes a double distillation column to concentrate HD from 290 ppm to approximately 4 mol %. A second double column concentrates HD in the upper column and recovers D2 and T2 as a side stream and a bottom output of the lower column. The HD exchange reaction was done in a separate fixed bed at non-cryogenic temperatures. Deuterium yield was less than 50%.
Oxygen exchange reactions are known in the literature and oxygen isotope separation by distillation is also known in the prior art. The oxygen exchange reaction is analogous to the H2 exchange reaction and can be described by the following equation:2(16O18O)=2(16O)+2(18O)
Naturally existing O2 contains 99.76% (16O), 0.21% (18O) and trace amounts of (17O).
Gorgoraki et al. (1964) studied oxygen exchange on zinc oxide at 79° K (see “A Study of the Homomolecular Exchange of Oxygen on ZnO at Low Temperatures” Kinetics and Catalysis 5(1), pp.120–127). Gorgoraki et al. reported stable activity over several hours and an apparent activation energy of 0.18 kcal/mol. This activation energy is comparable to that of the hydrogen exchange reaction at 22° K. The ZnO catalyst was treated in vacuo at 400° C. for 6 hours, then the reaction vessel was cooled to −194° C. A non-equilibrium mixture of isotopic oxygen was introduced at −194° C. The initial rate is very high but after 2 minutes, the activity becomes stable and unchanged for 3 hours. The rate of exchange was 0.026 e−4 mol/m2 hr.
Sazonov et al. (1966) studied oxygen exchange on gadolinium oxide (see “Homomolecular and Isotopic Exchange of Oxygen on Gadolinium Oxide” Kinetics and Catalysis 7(2), pp. 284–288). Sazonov et al. reported an increase in reaction rate from upon change in temperature from 242° K to 195° K. The rate at 195° K is equivalent to that measured on ZnO. Sazonov et al. suggest that the rate of exchange at low temperatures is a function of the amount of adsorbed oxygen on the catalyst, which increases with decreasing temperature.
SandIer et al., (1969) observed that under certain pretreatment conditions of oxidized palladium, the oxygen equilibrium is reached within 7 minutes at both 273° K and 195° K (“The Low-Temperature Isotopic Oxygen Equilibration on Oxidized Palladium” J. Phys. Chem. 73(7), pp. 2392–2396). This experiment was done with an oxygen pressure of 3.6 Torr (0.48 KPa). A different pretreatment scheme resulted in the exchange reaction half-life of 10 minutes at 195° K and 5.2 Torr (0.69 KPa).
U.S. Pat. No. 6,321,565 by Kihara et al., and EP 1092467 by Kihara et al., disclose cryogenic distillation of oxygen isotopes. A closed loop heat integration scheme using nitrogen, oxygen, air or ASU exhaust gas as heat transfer fluid is disclosed.
EP 1092467 discloses an isotope “scrambler” for improving isotope enrichment and is placed intermediate in the oxygen distillation sequence. The “scrambler” is used to temporarily convert enriched heavy oxygen gas to enriched heavy water by an oxidation reaction occurring at non-cryogenic temperatures under an argon atmosphere. Then, the enriched heavy water was immediately dissociated by electrolysis, scrambling the isotopic composition. The resulting heavy oxygen gas was fed to the next cryogenic distillation column in the sequence for further purification. The 18O yields were less than 10%.
Despite the foregoing developments, there is a need to provide an improved cost-efficient process of producing isotopes, particularly isotopes of hydrogen and oxygen in an improved yield.
All references cited herein are incorporated herein by reference in their entireties.